Andrew Yee[_1_]
January 9th 07, 05:15 AM
McDonald Observatory
University of Texas
Fort Davis, Texas
Contact: Rebecca Johnson
ph: 512-475-6763
fax: 512-471-5060
08 January 2007
University of Texas Astronomer's Studies of Galactic Bulges May Alter
Leading Theory of Galaxy Evolution
SEATTLE, Wash. -- David Fisher, an astronomy graduate student at The
University of Texas at Austin, is making important contributions to the
future understanding of galaxy evolution by studying the different types of
bulges at the hearts of nearby spiral galaxies. This work is being presented
this week at the 207th meeting of the American Astronomical Society in
Seattle, Washington.
A bulge is a concentration of stars in the center of a spiral galaxy. In
recent years, evidence has shown that there are two types of bulges -- the
so-called "classical bulges" and "pseudobulges."
Studying these bulges and finding out how many nearby galaxies have the
different types is important, Fisher says, because "we believe that the
formation of these two types of bulges is dramatically different." Such
findings could be important to theories of galaxy formation.
A classical bulge is a mostly featureless, round ball of stars, he says. A
pseudobulge, on the other hand, "looks very much like the [galaxy's] outer
disk, with a spiral structure. It can have a bar, and can be very flat,
instead of round."
Fisher and his colleague Niv Drory (Max-Planck-Institut f extraterrestrische
Physik) studied archival images of 40 galaxies within about 150 million
light-years from both Hubble Space Telescope and the Sloan Digital Sky
Survey. The archival Hubble images were of the very heart -- the bulge -- of
these galaxies. The Sloan images provided a look at the same galaxies in
their entirety -- providing information on the context for the bulge.
Fisher and Drory find that the global properties of galaxies are tightly
coupled to the type of bulge a galaxy contains, even when the bulge accounts
for only a few percent of the galaxy's mass.
This work is the basis for Fisher's doctoral dissertation. His goal is to
find a much more quantitative way to distinguish between the two bulge
types. His work presented at this conference shows that he is well on his
way. Once achieved, this would enable astronomers "to count how many of each
bulge type there are in the local universe," he says.
He explains that current theories of galaxy evolution focus on mergers, and
typically argue that the vast majority of nearby galaxies were built up
through mergers with other galaxies over time. Further, most galaxies are
predicted to have experienced a major merger in the past billion years.
Classical bulges are the result of mergers, Fisher says. "We feel
comfortable that they form through some kind of merger. You take two sets of
stars, you mix them up, and you're left with a ball that's essentially
featureless."
But pseudobulges form differently, through so-called "secular evolution."
Essentially, this means that the galaxy evolves on its own, without any
mergers. "In secular evolution," Fisher says, "the galaxy will start to
re-arrange itself, do it in an ordered way, rotating very fast. This causes
the spiral structure and nuclear bars in the center" that are seen in
pseudobulges. Pseudobulges are signposts of a history (since the formation
of the disk) that is free of mergers. Current theories of galaxy formation
do not predict this to be a common process. "We are trying to find out how
right or wrong these theories are," Fisher says.
An efficient method for counting the number of classical bulges and
pseudobulges in local galaxies will reveal which type is more prominent. If
there are a lot of pseudobulges, then there were not as many galactic
mergers in the past as astronomers think.
"The hints are there" that this is the case, Fisher says. If this turns out
to be correct, then "our galaxy formation models need more work."
-- END --
Note to Editors:
David Fisher can be reached this week in Seattle, and thereafter in Austin
at 512-471-1495.
University of Texas
Fort Davis, Texas
Contact: Rebecca Johnson
ph: 512-475-6763
fax: 512-471-5060
08 January 2007
University of Texas Astronomer's Studies of Galactic Bulges May Alter
Leading Theory of Galaxy Evolution
SEATTLE, Wash. -- David Fisher, an astronomy graduate student at The
University of Texas at Austin, is making important contributions to the
future understanding of galaxy evolution by studying the different types of
bulges at the hearts of nearby spiral galaxies. This work is being presented
this week at the 207th meeting of the American Astronomical Society in
Seattle, Washington.
A bulge is a concentration of stars in the center of a spiral galaxy. In
recent years, evidence has shown that there are two types of bulges -- the
so-called "classical bulges" and "pseudobulges."
Studying these bulges and finding out how many nearby galaxies have the
different types is important, Fisher says, because "we believe that the
formation of these two types of bulges is dramatically different." Such
findings could be important to theories of galaxy formation.
A classical bulge is a mostly featureless, round ball of stars, he says. A
pseudobulge, on the other hand, "looks very much like the [galaxy's] outer
disk, with a spiral structure. It can have a bar, and can be very flat,
instead of round."
Fisher and his colleague Niv Drory (Max-Planck-Institut f extraterrestrische
Physik) studied archival images of 40 galaxies within about 150 million
light-years from both Hubble Space Telescope and the Sloan Digital Sky
Survey. The archival Hubble images were of the very heart -- the bulge -- of
these galaxies. The Sloan images provided a look at the same galaxies in
their entirety -- providing information on the context for the bulge.
Fisher and Drory find that the global properties of galaxies are tightly
coupled to the type of bulge a galaxy contains, even when the bulge accounts
for only a few percent of the galaxy's mass.
This work is the basis for Fisher's doctoral dissertation. His goal is to
find a much more quantitative way to distinguish between the two bulge
types. His work presented at this conference shows that he is well on his
way. Once achieved, this would enable astronomers "to count how many of each
bulge type there are in the local universe," he says.
He explains that current theories of galaxy evolution focus on mergers, and
typically argue that the vast majority of nearby galaxies were built up
through mergers with other galaxies over time. Further, most galaxies are
predicted to have experienced a major merger in the past billion years.
Classical bulges are the result of mergers, Fisher says. "We feel
comfortable that they form through some kind of merger. You take two sets of
stars, you mix them up, and you're left with a ball that's essentially
featureless."
But pseudobulges form differently, through so-called "secular evolution."
Essentially, this means that the galaxy evolves on its own, without any
mergers. "In secular evolution," Fisher says, "the galaxy will start to
re-arrange itself, do it in an ordered way, rotating very fast. This causes
the spiral structure and nuclear bars in the center" that are seen in
pseudobulges. Pseudobulges are signposts of a history (since the formation
of the disk) that is free of mergers. Current theories of galaxy formation
do not predict this to be a common process. "We are trying to find out how
right or wrong these theories are," Fisher says.
An efficient method for counting the number of classical bulges and
pseudobulges in local galaxies will reveal which type is more prominent. If
there are a lot of pseudobulges, then there were not as many galactic
mergers in the past as astronomers think.
"The hints are there" that this is the case, Fisher says. If this turns out
to be correct, then "our galaxy formation models need more work."
-- END --
Note to Editors:
David Fisher can be reached this week in Seattle, and thereafter in Austin
at 512-471-1495.